今兒在逛論壇時(shí)，發(fā)現(xiàn)一個(gè)看似很簡(jiǎn)單的問(wèn)題，卻引起了大家的廣泛關(guān)注：
這是一道很早的面試題：
一張表,里面有ID自增主鍵,當(dāng)insert了17條記錄之后,刪除了第15,16,17條記錄,再把Mysql重啟,再insert一條記錄,這條記錄的ID是18還是15 。
答案：

如果表的類(lèi)型是MyISAM，那么是18。
因?yàn)镸yISAM表會(huì)把自增主鍵的最大ID記錄到數(shù)據(jù)文件里，重啟MySQL自增主鍵的最大ID也不會(huì)丟失。
如果表的類(lèi)型是InnoDB，那么是15。
InnoDB表只是把自增主鍵的最大ID記錄到內(nèi)存中，所以重啟數(shù)據(jù)庫(kù)或者是對(duì)表進(jìn)行OPTIMIZE操作，都會(huì)導(dǎo)致最大ID丟失。

自己也做了個(gè)實(shí)驗(yàn)，結(jié)果證實(shí)了上面的說(shuō)法。真是慚愧啊，看似簡(jiǎn)單的問(wèn)題自己也打錯(cuò)了。

復(fù)制代碼 代碼如下:

mysql> select * from test1;
+----+-----------+
| id | name|
+----+-----------+
| 1 | 陳兵輝 |
| 2 | chen|
| 3 | chen|
| 4 | chen|
| 5 | chen|
| 6 | chen|
| 7 | chen|
| 8 | chen|
| 9 | chen|
| 10 | chen|
| 11 | chen|
+----+-----------+
11 rows in set (0.00 sec)

mysql> delete from test1 where id in (10,11,9);
Query OK, 3 rows affected (0.03 sec)

mysql> show create table test1;
CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(10) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<SPAN>12</SPAN> DEFAULT CHARSET=utf8 |
mysql> exit;
Bye
[root@fsailing1 ~]# service mysqld restart
停止 MySQL：[確定]
啟動(dòng) MySQL：[確定]
[root@fsailing1 ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or /g.
Your MySQL connection id is 2
Server version: 5.0.95 Source distribution
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '/h' for help. Type '/c' to clear the current input statement.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show create table test1;
| CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(10) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<SPAN>9</SPAN> DEFAULT CHARSET=utf8 |

復(fù)制代碼 代碼如下:

mysql> select last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|0 |
+------------------+
1 row in set (0.00 sec)